Central necrosis following contusion to the sheep's spinal cord

The Translational Benefits of Sheep as Large Animal Models of Human Neurological Disorders

The past two decades have seen a considerable rise in the use of sheep to model human neurological disorders. While each animal model has its merits, sheep have many advantages over small animal models when it comes to studies on the brain. In particular, sheep have brains more comparable in size and structure to the human brain. They also have much longer life spans and are docile animals, making them useful for a wide range of in vivo studies. Sheep are amenable to regular blood and cerebrospinal fluid sampling which aids in biomarker discovery and monitoring of treatment efficacy. Several neurological diseases have been found to occur naturally in sheep, however sheep can also be genetically engineered or experimentally manipulated to recapitulate disease or injury. Many of these types of sheep models are currently being used for pre-clinical therapeutic trials, particularly gene therapy, with studies from several models culminating in potential treatments moving into clinical trials. This review will provide an overview of the benefits of using sheep to model neurological conditions, and highlight naturally occurring and experimentally induced sheep models that have demonstrated translational validity.

An ovine model of spinal cord injury

OBJECTIVE

To develop a large animal model of spinal cord injury (SCI), for use in translational studies of spinal cord stimulation (SCS) in the treatment of spasticity. We seek to establish thresholds for the SCS parameters associated with reduction of post-SCI spasticity in the pelvic limbs, with implications for patients.

STUDY DESIGN

The weight-drop method was used to create a moderate SCI in adult sheep, leading to mild spasticity in the pelvic limbs. Electrodes for electromyography (EMG) and an epidural spinal cord stimulator were then implanted. Behavioral and electrophysiological data were taken during treadmill ambulation in six animals, and in one animal with and without SCS at 0.1, 0.3, 0.5, and 0.9 V.

SETTING

All surgical procedures were carried out at the University of Iowa. The gait measurements were made at Iowa State University.

MATERIAL AND METHODS

Nine adult female sheep were used in these institutionally approved protocols. Six of them were trained in treadmill ambulation prior to SCI surgeries, and underwent gait analysis pre- and post-SCI. Stretch reflex and H-reflex measurements were also made in conscious animals.

RESULTS

Gait analysis revealed repeatable quantitative differences in 20% of the key kinematic parameters of the sheep, pre- and post-SCI. Hock joint angular velocity increased toward the normal pre-injury baseline in the animal with SCS at 0.9 V.

CONCLUSION

The ovine model is workable as a large animal surrogate suitable for translational studies of novel SCS therapies aimed at relieving spasticity in patients with SCI.

Degradation of cytoskeletal proteins in experimental spinal cord injury

Spinal cord injury was produced in rats by dropping a 10 g weight from 30 cm upon dura-invested exposed spinal cord. Examination of the fine structure of the traumatic lesion (15 min to 30 min) revealed granular degeneration of axons and occasional loosening of myelin lamellae. Older lesions (4 to 72 hours) showed degeneration of axons and vesiculation of myelin. At 15 minutes there is more loss of neurofilament proteins than of myelin proteins. Substantial decreases in the neurofilament and myelin proteins were observed at 30 minutes and the losses were even greater 2-72 hours after injury. This indicates that degeneration of axons may precede degradation of the myelin sheath and also that increased proteinase(s) activity, possibly activated by calcium, mediates the traumatic axonolysis and myelinolysis in experimental spinal cord trauma.